Variable pulse width parallel inverters



Jan. 22, 1963 D. v. JONES 3,075,136

VARIABLE PULSE WIDTH PARALLEL INVERTERS Filed Aug. 31, 1961 E LE; .1.

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United tatcs Patent 3,075,136 VARIABLE PULSE WIDTH PARALLEL INVERTERSDwight V. Jones, Baldwinsville, N.Y., assignor to General ElectricCompany, a corporation of New York Filed Aug. 31, 1961, Ser. No. 135,33411 Claims. (Cl. 321-45) This invention relates to parallel inverters andmore particularly to an improved arrangement of a parallel inverterwherein the pulse width of the inverter output can be readilycontrolled.

Parallel inverters employing such switching devises as thyra-trons,ignitrons and silicon controlled rectifiers, have been generally usedfor higher power applications as compared with the series type ofinverters. At low operating frequencies, the parallel inverter can beused to provide a substantially square-wave alternating output from adirect current source. By rectifying the output voltage of the parallelinverter, the inverter can be used as a DC. to D.C. converter or byemploying a recti-fied alternating voltage input, the inverter can bereadily adapted for use as a frequency changer.

In many applications of the parallel inverter, it is desirable that thepower output of the inverter be effectively regulated. For example, in alamp dimming system, a wide range of regulation is required toeffectively operate the lamps at various levels of luminous intensity.Similarly, in a power supply system for the operation of motors atvarious speeds, it is desirable, if not necessary, to regulate the powersupplied to the motors. Heretofore, the arrangements used to achievepower regulation in a parallel inverter have not been entirelysatisfactory. In prior art parallel inverter circuits employingthyratrons, regulation was achieved at the expense of increasedcommutating capacity, and the circuits were inefficient. There is aneed, therefore, for a parallel circuit inverter wherein a high degreeof regulation can be achieved readily and efficiently.

Accordingly,- an object of this invention is to provide an improvedvariable pulse width parallel inverter.

Another object of the present invention is to provide an improvedparallel inverter wherein regulation of the power supplied by theinverter is efiiciently achieved.

It is still another object of the invention to provide an improvedparallel circuit inverter wherein the pulse Width of the inverter outputcan be readily varied to achieve regulation by feedback controlarrangements.

The foregoing and other objects and advantages of the invention arerealized by a parallel inverter circuit having a pair of parallelconnected controlled rectifiers which are alternately triggered at apredetermined frequency to cause a reversal of current flow through afirst and a second primary winding portion of an output transformer. Theconduction time of the pair of controlled rectifiers is controlled byfiring a cut-off controlled rectifier at a predetermined point at eachhalf cycle to connect a charged capacitor in parallel with an inductorin the circuit and thus provide a pulse that turns off the conductingone of the pair of controlled rectifiers. In this manner the pulse widthof the inverter output is regulated. The firing of the cut-offcontrolled rectifier is synchronized with the start of the conduction ofthe controlled rectifiers so that the controlled rectifier is firedafter a predetermined interval in each half cycle after one of the pairof controlled rectifiers is fired.

The subject matter which I regard as my invention is set forth in theappended claims. The invention itself, however, together with furtherobjects and advantages thereof may be understood by referring to thefollowing description taken in connection with the accompanying drawingsin which:

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FIG. 1 is a schematic circuit diagram of a parallel inverter circuitillustrating one embodiment of the invention.

FIG. 2 is a schematic circuit diagram of the firing circuit fortriggering controlled rectifier SCR of the parallel inverter illustratedin FIG. 1; and

FIG. 3 is a schematic circuit diagram of the firing circuit used toalternately trigger controlled rectifiers SCR and SCR of the parallelinverter illustrated in FIG. 1.

Having more specific reference now to the parallel inverter identifiedgenerally by reference numeral 11 and shown in FIG. 1, it will he seenthat the parallel inverter '11 includes a pair of controlled rectifiersSCR, and SCR connected in circuit with a first winding portion P and asecond winding portion P of output transformer T As the controlledrectifiers SCR, and SCR are alternately triggered by firing circuit 12,a voltage of one polarity is induced across secondary windings S duringone half of each cycle of the inverter output and a voltage of oppositepolarity is induced across the secondary winding 8 in the other half ofeach cycle.

It will be noted that controlled rectifier SCR, and primary windingportion P are connected in a parallel circuit with controlled rectifierSCR and primary winding portion P across a direct current supply meanswhich includes a lead 13 connected to a center-tap 14 on primary windingP lead 15 connected to a ground 16, and input terminal lead 17 providedfor connection to the positive side of a direct current source.

The output transformer T has a magnetic core 18 and a secondary windingS inductively coupled With the pri-- mary winding P which is divided bytap 14 into primary winding portions P and P As shown in FIG. 1, thesecondary winding S is connected in circuit by output leads 1?, 2% witha load 21. Since transformer T is subjected to rapidly changing currentsduring the commutation interval of the parallel inverter 11, the leakagereactance of the transformer T was preferably kept down to a minimum.Thus, the transformer T was designed so that it operates belowsaturation. The leakage inductance between the primary winding portionsP and P and the source impedance is preferably kept relatively small sothat it will have no significant effect on the commutation of thecontrolled rectifiers SCR and SCR It will be seen that in theillustrative embodiment of the invention shown in FIG. 1, the inputterminal lead '17 is connected in circuit with a pair of feedback diodesD D and with controlled rectifiers SCR SCR through an inductor L Inputterminal lead 17 is provided for connection to a suitable direct currentvoltage source, which should be capable of accepting power as well assupplying power. Inductor L is connected in series circuit with acut-off controlled rectifier SCR A commutating capacitor C is connectedin parallel circuit across inductor L and controlled rectifier SCR sothat when the controlled rectifier SCR is triggered into a conductingstate, inductor L is connected in electrical circuit and in parallelwith the commutating capacitor C Thus, when connected in parallel,inductor L and the commutating capacitor C comprise an oscillatorycircuit.

Feedback diode D is connected in circuit with one end of primary windingportion P and in inverse relation with controlled rectifier SCRSimilarly, feedback diode D is connected in circuit with primary windingportion P of the primary winding P and in inverse relation withcontrolled rectifier SCR Connected in such an arrangement, the feedbackdiodes D D serve to limit the voltage across primary winding portions PP of transformer T to the magnitude of the DC. voltage supply and makeit possible to use silicon controlled rectifiers having a lowerbreakover voltage. Also, as will be hereinafter more full explained 3 inconnection with the description of the operation of the parallelinverter 11, the diodes D D feed back reactive power into the supply andthereby reduce voltage variations across the secondary winding SContinuing with the description of the circuit shown schematically inFIG. 1, it will be seen that one end of the inductor L is connected incircuit with commutating capacitor C and controlled rectifier SCR Theother end of the inductor L is connected in circuit with the anodes ofdiodes D D They are so poled that when either controlled rectifier SCRor SCR is in a conducting state, the lower plate of capacitor C as seenin the view of FIG. 1, will be negatively charged to a voltageconsiderably greater than the source of voltage. This increased voltageis due to the transformer action in charging the commutating capacitor Cthrough the diodes D and D and also the circuit time constants.Preferably, the inductor L provides an inductance sufiicient to keep thecurrent flow through it to a low value until the controlled rectifierSCR is turned off by the reversal of the current in the oscillatorycircuit comprised of the capacitor C and the inductor L The controlledrectifiers SCR SCR and SCR used in the illustrative embodiment of theinvention were PNPN semiconductors each having three terminals, an anoderepresented by the arrow symbol, a cathode represented by a line drawnthrough the apex of the arrow symbol and a gate represented by adiagonal line ex tending from the cathode. Silicon controlled rectifiersare desirable power switching devices since relatively large amounts ofpower can be switched into a load using an insignificant amount of powerto trigger the switching device. The operating characteristics of asilicon con trolled rectifier are such that it conducts in a forwarddirection with a forward characteristic very similar to that of anordinary rectifier when a gate signal is applied. Thus, when a positivevoltage is applied to the outside P layer and a negative voltage isapplied to the outside N layer, the two outside junctions are biased ina forward direction while the inner junction is reversely biased.Current does not flow through the controlled rectifier under theseconditions, except for a small leakage current. When the voltage isincreased to a breakover voltage, the current gain of the deviceincreases to unity at which time the current through the controlledrectifier will increase suddenly and become a function of the appliedvoltage and the load impedance. The controlled rectifier will remain ina conductive state provided the current through the device exceeds aminimum holding value.

A small amount of current supplied to the gate lead can be used forcontrolling the firing of the controlled rectifier since the currentsupplied to the gate lowers the breakover voltage. The controlledrectifier is normally operated well below the forward breakover voltageand is triggered by supplying current to the gate lead.

' A capacitor C is connected in circuit with the cathode of controlledrectifier SCR and across the primary winding portion P so that thecathode is clamped to ground when a negative turn-off pulse is appliedto the anode of controlled rectifiers SCR It also serves to minimizeswitching transients from the load. Similarly, capacitor C is connectedin circuit with controlled rectifier SCR and across primary windingportion P of the output transformer T so that the cathode of controlledrectifier SCR is clamped to ground when a negative turn-ofl pulse isapplied to its anode. Capacitors C and C enhance the performance of thecircuit, but they are not absolutely necessary since the circuit willoperate without them.

In order to provide a unidirectional supply voltage for firing circuit22, diodes D and D are connected across the primary winding P. A lead 23is connected in circuit with cathodes of the diodes D D the anodes beingconnected in circuit with the cathodes of conaovanae trolled rectifiersSCR SCR Thus, firing circuit 22 is energized when either controlledrectifier SCR or SCR is conducting. It will be seen that the diode D ispoled so that when controlled rectifier SCR starts conducting, a currentis supplied to firing circuit 22. Also, diode D is poled so that currentWill be supplied to firing circuit 22 the instant controlled rectifierSCR is triggered. In

- this manner the firing circuit 22 is synchronized with the start ofeach half cycle of the inverter output which begins with the triggeringof one of the controlled recti tiers SCR SCR In FIGS. 2 and 3, I haveillustrated the schematic circuit diagrams which are represented in theschematic diagram of FIG. 1 in block form. Firing circuit 12 isconnected in circuit with the gates and cathodes of controlledrectifiers SCR SCR by electrical leads 25, 26 and 27, 28, respectively.The gate and cathode of cutofi controlled rectifiers SCR are connectedin circuit with firing circuit 22 by means of electrical leads 29, 30.The type of firing circuits 12 and 22 which were employed in theexemplification of the invention shown in FIG. 1 are well known in theart and are described in the General Electric Controlled RectifierManual, first edition, 1960, at pages 50-58.

In accordance with the invention, the firing circuit 22 provides acurrent pulse to fire controlled rectifier SCR which turns oil theconducting controlled rectifier SCR or SCR at a predetermined point ineach half cycle. It will be seen that input lead 23 connects firingcircuit 22 in circuit with the controlled rectifiers so that a currentis supplied only when one of the controlled rectifiers is conducting. Aresistor R and the zener diode Z limit the maximum interbase voltage ofunijunction transistor UJT The zener diode Z is a semiconductor diode,preferably a silicon diode, having a predetermined reverse breakdownvoltage. For voltages below the breakdown value, the zener diode Z actsas a rectifier and only a negligibly small current can flow in thereverse direction. When the reverse voltage exceeds the breakdown value,the zener diode Z presents a very low resistance and permits current toflow freely in the reverse direction with no substantial increase involtage.

A resistor R is connected in circuit with the base-two electrode 31 inorder to compensate for temperature variations of the interbaseresistance of the unijunction transistor UJT Capacitor C is chargedthrough resistor R and the variable resistor R The rate at which thecapacitor C is charged to the peak emitter voltage of unijunctiontransistor UIT determines the point in each half cycle at whichunijunction transistor UJT is fired. When unijunction transistor UJT isfired, a pulse of current flows through primary winding P of the pulsetransformer T and a current pulse is induced in the secondary winding Selectrical leads 29, 30 applying this pulse across the gate and cathodeof cut-off controlled rectifier SCR It will be seen that the primarywinding P is connected at one end with base-one electrode 32 and at theother end with a ground 33.

Transistor Q acts as a's'hun-t to divert charging current flowingthrough the resistor R and R The amount of charging current diverted isproportional to the amount of current supplied to the base electrode ofthe transistor through lead 35. Thus, as base current is increased,additional current is diverted and the firing angle of the unijunctiontransistors UJT is retarded. Accordingly, the firing of controlledrectifier SCR is also retarded and the cut-off of the conductingcontrolled rectifier SCR o-r SCR is delayed.

.In FIG. 3, I have illustrated a transistor multi-vibrator firig circuit12 that was used in the illustrative embodiment of the invention toalternately apply firing pulses to controlled rectifiers SCR and SCR Twopairs of output leads 25, 26 and 27, 28 which are connected in circuitwith the secondary windings S S of pulse transformers T are provided forconnection across the gate and cathode of controlled rectifier-s SCR SCRrespectively, as shown in FIG. 1. The primary winding portions P P areconnected in circuit with the collector of transistors Q Q Inputterminal lead 36 is provided for connection to the positive side of thedirect current source used to energize the parallel inverter in FIG. 1.

Although in the exemplification of the invention a multi-vi-bratorcircuit configuration was used to provide alternating trigger pulseswith good symmetry to the controlled rectifiers SCR SCR it will beappreciated that other firing circuits can be used to generate andprovide alternate triggering pulses at a predetermined frequency to apair of controlled rectifiers. As an example, a pair of unijunctiontransistor relaxation oscillators coupled together by means of acapacitor connected between the emitters may be employed to provide thealternating pulses. Also, firing circuits utilizing saturating reactorsmay be used as a pulse source.

The unijunction relaXation oscillator portion of the multi-vibra-torcircuit provides good symmetry and frequency control to themulti-vibrator. Although in exemplification of the invention theinverter circuit ll shown in FIG. 1 was supplied with short durationtrigger pulses to fire controlled rectifiers SCR and SCR for highlyreactive loads the trigger pulse width must be extended in time for theduration of reactive current flow. The maximum pulse width at the gateconnot exceed the inverter pulse width, therefore for large reactiveloads the trigger pulse width must follow the changing inverter pulsewidth. This varying trigger pulse width can be obtained by varying thesymmetry of a multivibrator as a function of the inverter pulse width.

Continuing now with the description of the firing circuit 12 shownschematically in FIG. 3, it will be seen that the firing circuit 12employs two PNP transistors Q Q in a saturating flip-flop arrangement. Aunijunction transistor UJT serves to trigger the flip-flop from onestate to the other by providing a negative trigger pulse. This negativepulse is developed across resistor R and is coupled to the resistor R bymeans for the capacitor C Capacitor C serves as a timing capacitor.Cross coupling capacitors C and C are relatively small in size and areconnected in parallel with the cross-coupling resistors R R and incircuit with the base electrodes of transistors Q Q Diodes D and D clampthe base electrode of transistors Q Q to the emitter electrode.

Resistor R and the variable resistor or potentiometer R control thecharging rate or" the timing capacitor C and thereby serve as thefrequency control for the inverter circuit. Resistor R is connected tothe base-two electrode 37 of the unijunction transistor UIT Base-oneelectrode 38 is connected in circuit with a ground 39 by means oi leadsan, 41. Resistors R R R serve as temperature stabilizing resistors.Resistor R provides the frequency control for the output of the invertersince it controls the rate at which the flip-flop is triggered.Capacitors C and C in conjunction with the gate to cathode impedance ofthe controlled rectifier differentiate the square wave across secondarywindings S and S to provide a pulse output. Pulse triggering can be usedunless the load has a low and lagging power factor. Through the actionof the Zcner diode Z and resistor R a regulated DC. voltage is appliedto the unijunction transistor UJT Having reference now to the circuitshown in FIGS. 1, 2 and 3, the operation of the parallel invertercircuit ll will now be more fully described. When the positive terminalof a direct current source is connected in circuit with input terminallead 17 and the inverter circuit 11 is effectively grounded as shown inFIG. 1, inverter circuit 11 is energized. Let us assume arbitrarily thata firing pulse is supplied initially to the gate of the controlledrectifier SCR When controlled rectifier SCR is triggered, thecommutating capacitor C is charged to a voltage that is greater inmagnitude than the impressed voltage due to the transformed action thatcharges the capacitor C through the diode D Also, when controlledrectifier SCR is triggered, current flows through diode D to firingcircuit 22 and capacitor C begins its charging period.

Depending upon the setting of the variable resistor or potentiometer Rand the feed-back current being supplied to the transistor Q, theunijunct-ion transistor UJT will trigger the control rectifier SCR at apredetermined point in the half cycle of the alternating inverteroutput. When control-led rectifier SCR is triggered, it will be seenthat the lower plate of the commutating capacitor C is negativelycharged. Also, when controlled rectifier SCR is triggered, commutatingcapacitor C is connected in parallel circuit relation with the inductorL An oscillatory pulse is developed across the inductor L which reversebiases controlled rectifier SCR and thereby turns it ofi.

The commutating capacitor C will maintain a reverse bias acrosscontrolled rectifier SCR long enough for the controlled rectifier SCR toreturn to a blocking state. Capacitor C clamps the cathode of controlledrectifier SCR to ground 16 while the negative turn-off pulse is appliedto the anode. It will be seen that controlled rectifier SCR conducts fora very short interval since it is reverse biased when the currentreverses in the oscillatory circuit comprised of capacitor C and theinductor L During the commutating interval an inductive load connectedat the output of the inverter circuit 11 would prevent the main loadcurrent from reversing instantaneously. Diodes D D are thereforeprovided to feedback this current to the direct current supply until theload current reverses. In the first half of each cycle it will beappreciated that during the interval that current flows through diode Dcontrolled rectifier SCR will be back-biased, and if conducting, wouldbe turned off. For highly reactive loads, the triggering pulse widthprovided by fining circuit was extended in time for the duration of thereactive current flow but did not exceed the inverter pulse width.

Continuing with the description of the operation of the invertercircuit, the second half of the cycle com mences when a pulse issupplied to the gate of controlled rectifier SCR With controlledrectifier SCR conducting, the commutating capacitor C is charged throughdiode D the lower plate of capacitor C as seen in FIG. 1, again beingnegatively charged. The turn-off of controlled rectifier SCR isaccomplished in the same manner as the turn-oil of controlled rectifierSCR Firing circuit 22 triggers controlled rectifier SCR which connectsthe negatively charged plate of the commutating capacitor C in circuitwith the anode of controlled rectifier 'SCR and also connects thecapacitor C in parallel circuit with inductor L Thus, a negative pulseis developed across the inductor L which results in a reverse bias beingapplied across controlled rectifier SCR and it is turned oif. Similarly,diode D feeds back to the direct current source reactive power duringthe commutating interval, the amount of the feed-back being proportionalto the inductive components of the load.

It will be seen that as the firing circuit 12 applies positive triggerpulses alternately to the gates of the controlled rectifiers SCR SCR thecurrent from the direct current supply will flow alternately through theopposite ends of the primary winding P of the transformer T and generatean alternating current voltage across secondary winding S According tothe invention, regulation of the output across the secondary winding Sis achieved by turning oil the conducting controlled rectifier in eachhalf cycle to vary the pulse width of the inverter output.

From the foregoing description of the inverter circuit and itsoperation, it will be seen that the conduction time is regulated byfiring controlled rectifier SCR at a predetermined point in each halfcycle to connect the commutating capacitor C in parallel with theinductor L so that an oscillatory pulse is provided to turn oit theconducting controlled rectifier. The pulse width of the ontput voltagecan be readily varied in response to demands of the load thereto. Thecomponents used in the turn-01f circuit arrangement in accordance withthe invention have low power requirements since they handle only theturnoff energy for the controlled rectifier SCR SCR Further, thearrangement in accordance with the invention is readily adaptable to afeed-back type of control.

It will be understood that the preferred embodiment of the inventiondescribed herein is intended as an illustrative example of the inventionand that the invention is not necessarily limited to such an embodimentthereof. It will be apparent that many modifications of the inventiondescribed herein may be made. As, for example, the inverter circuit maybe modified so that the positive terminal and negative terminalconnection of the power sup ply are reversed. Further, the invertercircuit arrangement of the invention is adaptable to three phaseapplications. It is to be understood, therefore, that I intend by theappended claims to cover all such modifications that fall within thetrue spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a parallel inverter having a first controlled rectifier and afirst primary winding portion connected in parallel circuit relationwith a second controlled rectifier and a second primary winding portionacross a direct current supply means, an inductor connected in circuitwith said first and second controlled rectifier, a commutatingcapacitor, a cut-off controlled rectifier, circuit means connecting saidcut-off controlled rectifier in series circuit relation with saidinductor and in parallel circuit relationship with said commutatingcapacitor, a first firing circuit means to alternately fire said firstand second controlled rectifier, a second firing circuit means connectedin circuit with said cut-E controlled rectifier to fire said cut-oficontrolled rectifier at a predetermined point in each half cycle aftersaid first and second controlled rectifiers are fired, said cut-oifcontrolled rectifier when triggered causing an oscillatory pulse to beinduced across said first inductor thereby turning oif the conductingone of said first and second controlled rectifiers.

2. The inverter circuit as set forth in claim 1 wherein said secondfiring circuit means includes a first and a second diode, each of saiddiodes having an anode and a cathode, the anode of said first diodebeing connected in circuit with the cathode of said first controlledrectifier and the anode of said second diode being connected in circuitwith the cathode of said second controlled rectifier, the cathodes beingconnected in circuit with a unijunction transistor oscillator andcausing said oscillator to be energized only when one of said first andsecond controlled rectifiers are in a conducting state therebysynchronizing said second firing circuit means with said first andsecond controlled rectifiers.

3. The inverter circuit as set forth in claim 1 wherein a capacitor isconnected across each of said first and second primary winding portionsin order to clamp the cathodes of said first and second controlledrectifiers to ground when a negative oscillatory pulse is applied to theanodes thereof.

4. In a parallel inverter having a pair of controlled rectifiers and atransformer with a secondary and a first and a second primary windingportion, one of said controlled rectifiers and said first windingportion being connected in parallel circuit relation with the other ofsaid controlled rectifiers and said second winding portion and incircuit with a direct current supply means, a cut-off controlledrectifier having an anode, a cathode and a gate electrode, a firstinductor connected in circuit with said pair of controlled rectifiers, acommutating capacitor connected in series circuit relation with thecathode of said cut-ofi controlled rectifier and in circuit with saidfirst inductor so that when said cut-off controlled rectifier istriggered said commutating capacitor is connected in parallel circuitrelation with said first inductor, said first inductor and saidcapacitor comprising an oscillatory circuit, a second inductor connectedin circuit with said commutating capacitor, 21 pair of diodes, circuitmeans :triggering said cut-ofi controlled rectifier at a predeterminedpoint in each half cycle, circuit means alternately triggering one ofsaid pair of controlled rectifiers into a conducting state to provide analternating output across the secondary of said transformer, saidcut-ofif controlled jrectifier when in a conducting state connectingsaid commutating capacitor in parallel circuit with said first inductorthereby producing an oscillatory pulse across said inductor to turn oifthe conducting one of said pair of controlled rectifiers.

5. A parallel inverter comprising a transformer having a secondarywinding and a primary winding split into a first primary winding portionand a second primary winding portion, a first controlled rectifierconnected in series circuit relation with said first winding portion, asecond controlled rectifier connected in series circuit relation withsaid second winding portion, a direct current supply means, saidserially connected first controlled rectifier and first winding portionbeing connected in parallel circuit relation with said seriallyconnected second controlled rectifier and second winding portion, firingcircuit means connected in circuit with said first and second controlledrectifiers to cause a current to flow in one direction through saidfirst Winding portion in one half cycle and in an opposite directionthrough a second winding portion in the other half cycle, a turn-oilcontrolled rectifier having an anode, a cathode and a gate, an inductivemeans connected in circuit with said first and second controlledrectifiers and in circuit with the anode of said cut-ofi controlledrectifier, a capacitor connected in cir cuit with the cathode of saidcut-ofi controlled rectifier, said capacitor and said inductive meanscomprising an oscillatory circuit, circuit means connecting saidcapacitor in circuit with said first and second controlled rectifier sothat said commutating capacitor is charged in each half cycle when oneof said first and second controlled rectifiers is conducting, firingcircuit means connected to the gate of said cut-off controlled rectifierand firing said cut-0E controlled rectifier at a predetermined point ineach half cycle after one of said first and second controlled rectifiersis fired to control the pulse width of the output across the secondarywinding of said transformer.

6. In a parallel inverter circuit having a transformer with a firstprimary winding portion and a secondary primary winding portioninductively coupled with a secondary winding, a pair of controlledrectifiers, each of said controlled rectifiers having an anode, acathode and a gate, one of said controlled rectifiers and said firstprimary winding portion being connected in parallel with the other ofsaid controlled rectifiers and said second primary winding portion andin circuit with a direct current supply means, a cut-off controlledrectifier having an anode, a cathode and a gate, an oscillatory circuitmeans connected in circuit with the anodes of said pair of con- ,trolledrectifiers and in circuit with said cut-ofi controlled rectifier, saidcut-otf controlled rectifier when trig- ;gered causing a negativeoscillatory pulse to be prorectifiers and thereby regulate the pulseWidth of the output of said parallel inverter.

7. A parallel inverter having a transformer with a split primary dividedinto a first primary winding portion and a second primary windingportion inductively coupled with a secondary winding, a direct currentsupply means, and a pair of controlled rectifiers, each of saidcontrolled rectifiers having an anode, a cathode and a gate, said pairof controlled rectifiers being alternately fired at a predeterminedfrequency to provide an alternating output across said secondarywinding, one of said controlled rectifiers and said first windingportion being connected in parallel circuit relation with the other ofsaid controlled rectifiers and said second primary Winding portionacross said direct current supply means, a cut-off controlled rectifierhaving an anode, a cathode and a gate electrode, an inductor connectedin circuit with the anodes of said pair of controlled rectifiers and incircuit with the anode of said cut-off controlled rectifier, a capacitorconnected in circuit across said inductor and said cut-off controlledrectifier, a pair of diodes, said diodes being connected in circuit withsaid capacitor and the cathodes of said pair of controlled rectifiers sothat when one of said pair of controlled rectifiers is fired saidcapacitor is charged, firing circuit means firing said cut-oifcontrolled rectifier at a predetermined point in each half cycle tocut-off the conducting one of said pair of controlled rectifiers therebyregulating the pulse width of the inverter output.

8. In an inverter circuit having a transformer with a primary windingsplit into a first primary Winding portion and a second primary windingportion, a direct current supply means, a pair of controlled rectifiers,one of said controlled rectifiers and said first primary winding portionbeing connected in parallel circuit relation with the other of saidcontrolled rectifiers and said second primary winding portion in circuitwith the direct current supply means, a cut-off controlled rectifierhaving an anode, a cathode and a gate, and a first inductor connected inseries circuit with the anodes of said pair of controlled rectifiers andthe anode of said cut-01f controlled rectifier, a capacitor connected incircuit across said first inductor and said cut-off controlledrectifier, said first inductor and said capacitor comprising anoscillatory circuit, a second inductor connected at one end with thecathode of said cut-ofi controlled rectifier and in circuit with saidcapacitor, a pair of diodes, each having an anode and cathode, theanodes of said diodes being connected in circuit with said secondinductor and the cathodes being connected in circuit across said primarywinding of said transformer and firing circuit means to trigger said cu-oif controlled rectifier at a predetermined point in each half cycleduring the conduction period of said pair of controlled rectifiers toturn off the conducting one of said pair of controlled rectifiers andthereby regulate the pulse width of said inverter output.

9. A parallel inverter comprising a transformer having a primary Windingwith a central tap dividing the primary winding into a first windingportion and a second winding portion, and a secondary windinginductively coupled with the primary winding on a magnetic core, saidtap being provided for connection to ground, a pair of controlledrectifiers, each of said controlled rectifiers having an anode, acathode and a gate, the cathode of one of said controlled rectifiersbeing connected in circuit with one end of said first winding portionand the cathode of the other of said controlled rectifiers beingconnected in circuit with the other end of said primary Winding, adirect current supply means, an inductor having one end connected incircuit with the anodes of said controlled rectifiers, and the other endthereof connected in circuit with said direct current supply means, afirst diode connected in inverse relation across said first controlledrectifier and said inductor, a second diode connected in inverserelationship across said second controlled rectifier and said inductor,a cut-off controlled rectifier having an anode, a cathode and a gate, asecond inductor, said cut-off controlled rectifier having its anodeconnected in circuit with said first inductor and its cathode in circuitwith one end of said second inductor, a pair of diodes connected incircuit with the other end of said second inductor, each of said diodeshaving an anode and cathode, the cathode of one of said aforementioneddiodes being connected in circuit With the cathode of said firstcontrolled rectifier and the cathode of the other of said aforementioneddiodes being connected in circuit with the cathode of said secondcontrolled rectifier, a capacitor connected in circuit across said firstinductor and said cut-off controlled rectifier, said capacitor and firstinductor comprising an oscillatory circuit, firing circuit meansconnected in circuit with the gates of said first and second controlledrectifiers to alternately fire said controlled rectifiers to provide anoutput voltage of alternating polarity across the secondary winding ofsaid transformer and a second firing circuit means connected in circuitwith the gate and cathode of said cut-ofi controlled rectifier to firesaid cut-off controlled rectifier at a predetermined point in each halfcycle to cause said first inductor and said capacitor to be connected inparallel thereby resulting in an oscillatory pulse across said firstinductor, said oscillatory pulse causing the conducting one of saidfirst and second controlled rectifiers in each half cycle to be turnedoff thereby regulating the pulse width at the inverter output.

10. In a parallel inverter apparatus having a pair of controlledrectifiers connected in parallel across the primary winding of atransformer to cause a direct current to be alternately dischargedthrough a first and a second primary winding portion, an inductorconnected in circuit with the controlled rectifiers, a turn-offcontrolled rectifier having an anode, a cathode and a gate electrode, afiring circuit means triggering said turn-off controlled rectifier at apredetermined point in each half cycle, a commutating capacitor, circuitmeans connecting said inductor, said turn-otf controlled rectifier andcommutating capacitor so that when said turn-ofi controlled rectifier isin a conducting state said inductor is connected in parallel circuitrelation with said commutating capacitor, said inductor and saidcommutating capacitor comprising an oscillatory circuit, said conductingcontrolled rectifier in each half cycle being turned ofi by the resonantpulse produced across said inductor and said turn-off controlledrectifier being turned 01f when the current reverses in said oscillatorycircuit. l

11. A parallel inverter circuit comprising a transformer having aprimary winding and a secondary winding, a tap on said primary Windingdividing said primary winding into a first and a second Winding portion,said secondary winding having a pair of output terminal leads, a pair ofcontrolled rectifiers, each of said controlled rectifiers having ananode, a cathode and a gate, firing circuit means connected in circuitwith said gate and cathode of said controlled rectifiers to alternatelytrigger said controlled rectifiers at a predetermined frequency, one ofsaid controlled rectifiers being triggered at the start of the firsthalf of each cycle and the other of said controlled rectifiers beingtriggered at the start of the second half of each cycle, a directcurrent supply means, circuit means connecting said controlledrectifiers, said direct current supply, said tap in circuit with saidfirst and second winding portions so that when one of said controlledrectifiers is triggered a current is caused to flow in the first Windingportion in one direction and when the other of said controlledrectifiers is triggered a current is caused to flow in an oppositedirection in the second winding portion thereby producing an alternatingcurrent in the secondary winding of said transformer, an inductorconnected in circuit with said controlled rectifiers, a capacitor, aturn-off controlled rectifier, a firing circuit means triggering saidturn-off controlled rectifier at a predetermined point in each halfcycle thereby regulating the pulse width of the inverter output, andcircuit means connecting said inductor, said turn-off controlledrectifier and said capacitor in circuit so that When said controlledrectifier is triggered said capacitor is connected in parallel circuitwith said inductor and in circuit with the conducting controlledrectifier to reverse bias said conducting one of said pair of controlledrectifiers, said inductor and commutating 12 capacitor forming anoscillatory circuit when said turnoff controlled rectifier is conductingand the reversal of current in said oscillatory circuit causing saidturn-01f controlled rectifier to be turned ofi.

References Cited in the file of this patent UNITED STATES PATENTS3,047,789 Lowry July 31, 1962

4. IN A PARALLEL INVERTER HAVING A PAIR OF CONTROLLED RECTIFIERS AND ATRANSFORMER WITH A SECONDARY AND A FIRST AND A SECOND PRIMARY WINDINGPORTION, ONE OF SAID CONTROLLED RECTIFIERS AND SAID FIRST WINDINGPORTION BEING CONNECTED IN PARALLEL CIRCUIT RELATION WITH THE OTHER OFSAID CONTROLLED RECTIFIERS AND SAID SECOND WINDING PORTION AND INCIRCUIT WITH A DIRECT CURRENT SUPPLY MEANS, A CUT-OFF CONTROLLEDRECTIFIER HAVING AN ANODE, A CATHODE AND A GATE ELECTRODE, A FIRSTINDUCTOR CONNECTED IN CIRCUIT WITH SAID PAIR OF CONTROLLED RECTIFIERS, ACOMMUTATING CAPACITOR CONNECTED IN SERIES CIRCUIT RELATION WITH THECATHODE OF SAID CUT-OFF CONTROLLED RECTIFIER AND IN CIRCUIT WITH SAIDFIRST INDUCTOR SO THAT WHEN SAID CUT-OFF CONTROLLED RECTIFIER ISTRIGGERED SAID COMMUTATING CAPACITOR IS CONNECTED IN PARALLEL CIRCUITRELATION WITH SAID FIRST INDUCTOR, SAID FIRST INDUCTOR AND SAIDCAPACITOR COMPRISING AN OSCILLATORY CIRCUIT, A SECOND INDUCTOR CONNECTEDIN CIRCUIT WITH SAID COMMUTATING CAPACITOR, A PAIR OF DIODES, CIRCUITMEANS CONNECTING SAID DIODES IN CIRCUIT WITH SAID PAIR OF CONTROLLEDRECTIFIERS AND SAID COMMUTATING CAPACITOR SO THAT SAID CAPACITOR ISCHARGED DURING EACH HALF CYCLE WHEN ONE OF SAID FIRST AND SECONDCONTROLLED RECTIFIERS IS CONDUCTING, FIRING CIRCUIT MEANS CONNECTED WITHTHE GATE OF SAID CUT-OFF CONTROLLED RECTIFIER, SAID FIRING CIRCUIT MEANSTRIGGERING SAID CUT-OFF CONTROLLED RECTIFIER AT A PREDETERMINED POINT INEACH HALF CYCLE, CIRCUIT MEANS ALTERNATELY TRIGGERING ONE OF SAID PAIROF CONTROLLED RECTIFIERS INTO A CONDUCTING STATE TO PROVIDE ANALTERNATING OUTPUT ACROSS THE SECONDARY OF SAID TRANSFORMER, SAIDCUT-OFF CONTROLLED RECTIFIER WHEN IN A CONDUCTING STATE CONNECTING SAIDCOMMUTATING CAPACITOR IN PARALLEL CIRCUIT WITH SAID FIRST INDUCTORTHEREBY PRODUCING AN OSCILLATORY PULSE ACROSS SAID INDUCTOR TO TURN OFFTHE CONDUCTING ONE OF SAID PAIR OF CONTROLLED RECTIFIERS.